Convert RETAIL to OEM

I'm if I could convert my already installed retail version to oem one?
The current activation key is only for OEM Channel I'm afraid...
I've found that in a voatk's Vista activator there is this option, do you know how to do it on windows 7?

As I saw already in voatk, it replaces the tokens.dat and may be it's doing something else, I don't know...